US secretary of state says that "time and time again" Syria's Assad has failed to live up to promise to end fighting.

At a joint press conference in Brussels held Wednesday by US Secretary of State Hilary Clinton and US Secretary of Defense Leon Panetta, Clinton said that the "burden has shifted" to those supporting the regime of Syrian President Bashar Assad to explain how they can permit the ongoing violence.

Violence has continued virtually unabated for over a year in Syria between the government and Syrian rebels; a ceasefire deal signed last week has ushered in a reduction of violence, though government forces have been maintaining military presence in civilian zones.

Clinton reemphasized a mantra of the US government that she would judge Syria going forward "by its actions, not its words."

The secretary of state said that "time and time again" Syria has failed to live up to its commitments to end the fighting despite its own guarantees and the guarantees of its backers.

Russia and China have held back most attempts to sanction or take other UN action against the Syrian regime to pressure it into stopping its assault against the opposition.

Clinton refrained from promising to provide arms to the Syrian opposition, but pointed out that the US has continued to provide them logistical support.

Clinton expressed alarm that the conflict was impacting nations outside Syria's borders, including Turkey and Lebanon, where Syrian troops allegedly shot at people fleeing across the borders in recent incidents.

The US hopes to support the Syrian opposition in becoming the voice of "Syria's future," she said.